When a signal transmitted by a transmitting station is received by a mobile device, in general, the faster the velocity of motion of the mobile device is, the greater the signal degradation due to fading becomes, making the desired receiving performance difficult to obtain. Transmitting and receiving methods that reduce degradation in the quality of the demodulated signal are therefore applied, e.g., by employing a modulation system with high fading immunity, a typical method being DQPSK (Differential Quadrature Phase Shift Keying), or by applying waveform equalization techniques with high resistance to fading in the receiver.
Another proposed technique is to improve reception performance by controlling demodulation operations according to the velocity of motion. For example, a method proposed in patent documents 1 and 2 is to detect velocity of motion by using the GPS (Global Positioning System), a gyroscope, a velocimeter, or the like, and control demodulation operations according to the result.
Another proposed technique utilizes the fact that the velocity of motion of a mobile device, and changes in that velocity appear in the received signal as temporal variations in channel characteristics, to detect the velocity of motion of the mobile device from the received signal, and to optimize demodulation operations in the reception device according to the velocity information, thereby to improve the reception performance.
For example, in demodulating the synchronously modulated subcarriers used in the QPSK (Quadrature Phase Shift Keying) and QAM (Quadrature Amplitude Modulation) systems, a reception device (see, for example, patent document 3) for receiving the orthogonal frequency-division multiplexed signals employed in terrestrial digital broadcasting or the like estimates the amplitude and phase variations of each subcarrier that occur in the channel (this will also be referred to below as channel estimation) by using a known signal (also referred to below as a pilot signal) inserted into the transmitted signal, and adjusts the amplitude and phase of the subcarriers on the basis of the estimation result (this will also be referred to below as equalization).
To optimize demodulation operations by tracking temporal variations in channel characteristics, a proposed system detects the amplitude variations in the received signal from a CP (Continual Pilot), TMCC (Transmission and Multiplexing Configuration Control), or AC (Auxiliary Channel) signal or some other signal inserted into the transmitted signal at a fixed amplitude, and controls the channel estimation method according to the result (see, for example, patent document 4).
Furthermore, patent document 5 describes a method of detecting velocity of motion from amplitude and phase variations in a pilot signal.